Added environment based shortcuts in symfony2 plugin

plugins/symfony2/symfony2.plugin.zsh

@@ -25,3 +25,5 @@ alias sfcw='sf cache:warmup'
 alias sfroute='sf router:debug'
 alias sfcontainer='sf container:debug'
 alias sfgb='sf generate:bundle'
+alias dev="sf --env=dev"
+alias prod="sf --env=prod"